Rechargeable batteries manufactured from laminates of solid polymer electrolytes and thin films anodes and cathodes displays many advantages over conventional liquid electrolytes batteries. These advantages include lower overall battery weight, high power density, high specific energy, longer service life, and environmentally friendly since the danger of spilling toxic liquid into the environment is eliminated.
Solid polymer battery components include positive electrodes, negative electrodes and an insulating material capable of permitting ionic conductivity such as a solid polymer electrolyte sandwiched between the electrodes. The anodes or negative electrodes are usually made of lightweight metals films, such as alkali metals and alloys typically Lithium metal, lithium-aluminum alloys and the like. The composite cathodes or positive electrodes are usually formed of a mixture of active material such as transitional metal oxide, an electrically conductive filler, usually Carbon particles, an ionically conductive polymer electrolyte material and a current collector usually a thin sheet of aluminum. Composite cathode thin films are usually obtained by coating onto a current collector.
The production of thin films of lithium having a thickness lower than 75 mircrometers and in the form of wide bands, for example 10 centimetres or more and in lengths of many hundreds of meters, by means of rapid and reliable processes, faces important technical difficulties which are attributable to the extreme physical and chemical properties of this metal such as chemical reactivity, malleability, low mechanical strength, rapid self-welding by simple contact and strong adhesion on most solid materials, for example the usual metals.
This difficulty is confirmed by the difficulty of obtaining from suppliers of specialty metals and chemical products, thin lithium films 40 micrometers (μm) thick and less, of sufficient surface and length, having an adequate surface finish and chemical property to be used in lithium cells.
Presently, cold extrusion is used for the continuous production of sheets 75 μm and more. These thicknesses are generally adapted to the production of lithium cells utilizing liquid electrolytes. For lower thicknesses, the films obtained by extrusion are thereafter laminated between rollers made of hard materials. These processes have been described and are commercially used for the production of limited quantities of sheets of 30-75 microns.
U.S. Pat. No. 3,721,113, describes a process in which multiple successive lamination passes between steel rollers protected by films of hard plastic which are non reactive towards lithium are required to give thin films of 30-40 μm. U.S. Pat. No. 4,824,746 describes an alternative process based on the coating of molten lithium on a metallic of plastic support.
In large scale production processes, the difficulties in achieving the efficient lamination of dense lithium to a thickness varying between 15 and 40 microns for the production of polymer electrolyte cells are numerous: First, the laminated lithium metal often reacts with, and/or deforms, and adheres to the lamination rollers with which it is in contact. This problem has been solved by the use of a lubricating agent as described in U.S. Pat. Nos. 5,837,401, 5,528,920 and 6,019,801. The lubricating agent comprises additives which prevent the thin laminated lithium film from reacting or excessively adhering to the lamination rollers and does not effect the electrochemistry of the resulting electrochemical cell. Secondly, the extreme ductility of lithium or alloys thereof allows only minuscule drawing tension on the lithium film exiting the lamination rollers. For example, a film of lithium breaks under a drawing tension higher than 579.13 Kpa, its low limit of elasticity. The drawing tension must therefore be precisely monitored and controlled to prevent breakage or ripping of the lithium film and consequently, costly interruptions of production.
Thirdly, with a thickness between 5 to 50 microns, it is difficult to laminate a film of lithium or alloy thereof to a constant thickness across the entire width of the film. Variations of thickness occurs across the width of the laminated lithium film in conventional lamination processes which promote breakage of the lithium film during the lamination operation and renders the resulting laminated lithium film less than adequate for electrochemical cells.
Thus there is a need for a method and apparatus adapted to laminate a thin film of lithium or lithium alloy in a single pass that alleviates the limitations of the prior art and that provides a film of constant thickness.